1. Field of the Invention
This invention relates to monitoring systems, and more particularly, to an apparatus that monitors the vital signs, i.e., pressure, pulse rate, respiration rate and temperature of an individual.
2. Description of the Prior Art
With respect to blood pressure, there are two major measuring methods commonly used: (1) Auscultory method; (2) Oscillometric method. The auscultory method uses a piezoelectric transducer, which is inserted in a cuff to be positioned on the brachial artery of the person whose vital signs are being monitored. Because piezoelectric transducers respond to sound waves, sound signals from this artery are detected corresponding to first and last pressure pulses. These pulses are thereafter processed to determine systolic and diastolic pressures, respectively. The main disadvantage of this method is the critical positioning of the transducer on the brachial artery. If not properly positioned, the blood pressure readings are consequently affected. If the operator is untrained, careless, or does not realize the importance of correct placement, incorrect readings are likely to result.
The oscillometric method constitutes an improvement over the auscultory method because cuff positioning around the arm does not require any special precautions. The cuff is employed to transmit pressure pulses to a transducer located within a monitoring apparatus, there being no transducer in the cuff itself. In using this method, the first detectable systolic pressure pulse and the largest pressure pulse at the lowest cuff pressure corresponding to the mean arterial pressure is detected. From this value the diastolic pressure is computed by complex equations. In order to obtain the systolic pressure pulse and the largest pressure pulse, the stop-deflate approach is normally used. In this approach, the cuff is inflated to a certain known pressure level, such as 170 mmhg. The cuff is then deflated approximately 7 mmhg and the deflation is stopped to search for pressure pulses. In the absence of pressure pulses, the deflation-stop routine is repeated, again and again. Eventually, the first recognizable systolic and the largest pressure pulse is detected. Using the cuff pressure value at which time these values occur, diastolic pressure is determined. A significant disadvantage of this approach is that the computed and the actual diastolic pressures do not closely coincide. Furthermore, it is undesireable for a person whose vital signs are being monitored to have one arm under pressure for prolonged periods of time. It is also important to note that in cases where the pressure pulses of the individual whose vital signs are being monitored are all of low amplitude, it is very difficult to detect a maximum amplitude pulse from among all the pulses. Thus, the mean pressure upon which all succeeding calculations are based is very likely to be incorrect. Even more important to note is the fact that prior art devices detect the systolic and mean arterial pressures first, and then calculate the diastolic pressure from said systolic and mean arterial pressure. Since the systolic and mean arterial pressure detection are often erroneous, and since the diastolic pressure is calculated from these values, it is clear that errors are added upon errors in arriving at the diastolic pressure determination. This is highly objectionable, because the diagnosis of ailments of the heart and kidney, for example, rely heavily on diastolic pressure values.
A better approach using this method is to detect the first and last pressure pulses corresponding to the true systolic and diastolic pressures as the cuff is continuously deflated without interruption. However, an awkward problem is encountered in this approach while detecting pressure pulses with audible sound. The audibility of pressure pulses is affected by the pulse amplitude as well as by the rigidity of the brachial artery wall. In some persons with hardened arterial walls, the pressure pulses generate audible sound although the pulse amplitude is very small. In contrast, other persons with very smooth and flexible arterial walls require very large amplitude pressure pulses to generate audible sound.
There is thus a need for a vital sign monitor with the capability of accurately determining pressure values on persons with all kinds of arterial wall conditions.
It is therefore an object of this invention to provide a vital sign monitor that computes pressure, pulse, respiration and temperature and which displays the values automatically.
It is another object of the present invention to provide a vital sign monitor that computes accurately and very quickly blood pressure values of any person regardless of age, size, height and artery condition.
It is still another object of the present invention to provide a vital sign monitor with sufficient sensitivity to detect any range of pressure pulses in any individual.
It is a further object of the invention to provide a vital sign monitor that computes systolic and diastolic pressures by using continued and uninterrupted cuff deflation, thereby assuring speed and comfort to the person whose vital signs are being monitored.